1. Field of Invention
This invention relates to a synchronous motor for a timepiece and in particular to a synchronous motor for a timepiece which converts an alternating electric signal from a time signal generating means to mechanical movement of constant speed rotation.
2. Description of the Prior Art
In order to convert time generating signals of pulses, etc. supplied by an AC commercial power source, crystal oscillator, or other oscillating means with highly accurate frequency to mechanical rotation of time indicating hands; various synchronous motors for timepieces find wide utility in accurate analog type display timepieces. For this type of synchronous motor, small electric power consumption and reliable self-starting properties are demanded. The conventional synchronous motor, however, could not completely meet the demand of these requirements.
In FIG. 1, shown therein is a conventional synchronous motor, which is composed of a rotor 10, stator plates 12 and 14, and an energizing coil 16. The rotor 10 has two rotor poles which are shown as N and S poles magnetized in opposite positions at 180 degrees from each other in the Figure. On the other hand, the stator plates 12 and 14 have a pair of stator poles on each stator plate, 12a and 12b, or 14a and 14b, arranged at 90 degrees respectively, and the stator poles of 12a and 14a, and 12b and 14b are arranged across from each other against the center of the rotor 10. At the common stem of the stator plates 12 and 14 the energizing coil 16 is windingly arranged to be supplied an alternating electric signal from a time signal generating means and alternating magnetic flux is provided to the stator composed of the respective stator plates of 12 and 14. This alternating magnetic flux generates oscillating movement in the form of electro-magnetic attractive and repelling forces over the rotor 10. When the oscillating movement grows gradually to exceed a specific oscillating angle, the rotor 10 starts the movement at a constant speed rotation synchronized with the input alternating electric signal. The value of the constant speed rotation is determined by the number of rotor and stator poles and the frequency of the alternating electric signal, and movement of mechanical rotation with high accuracy is provided depending on the accuracy of the alternating electric signal supplied to the energizing coil 16 when the rotor 10 is connected and arranged to the well-known time indicating gear train, which makes it possible to provide a analog type display timepiece with time indicating hands operated by the time indicating gear train.
The prior art device in FIG. 1, however, has such drawbacks that the existance of an unbalanced state in the driving power of self-starting deteriorates the self-starting power properties and results in the loss of efficiency of the motor, since the rotor 10 has two static central positions which are basically located against the dynamic magnetic center line of the stator. The static central position of the rotor shows the standstill position of the rotor 10 by the line connecting the facing rotor poles in such state that the signal is not supplied to the energizing coil and that no alternating magnet flux is produced to the respective stator plates 12 and 14. In FIG. 1, it is shown as the location that two rotor poles N and S coincide with the line connecting the opposite stator poles either 12a and 14a or 12b and 14b, that is, it is shown by the straight line A or B. In another words, in the static central position A or B, the rotor poles are attracted by the stator poles and the rotor is caused to stand still. In the prior art device, it is not defined which of the static central position A or B the rotor 10 takes, but either static central position can be selected with same probability resulting in standstill.
On the other hand, the dynamic magnetic center line, that is, the strongest position of the electro-magnet coupling between the stator and the rotor at the state that the coil 16 is excited and the alternating magnetic flux is supplied to the stator, is shown by a line C in the FIG. 1.
Each static central position A or B and the dynamic magnetic center line C are not geometrically crossed with the dynamic magnetic center line at right angles to produce an unbalanced state in driving power at the time of self-starting. Taking the example of the static central position B, it is well-understood to produce such remarkably unbalanced state in the driving power that there exists an offset angle of 45 degrees clockwise or 135 degrees counter-clockwise against the dynamic magnetic center line C, which makes a massive difference in the strength of initial oscillating movement towards a certain rotating direction in starting the rotor and results in the loss of growing speed in the oscillating movement.
Also in the prior art device, the number of rotor poles is set at two, and large initial oscillation exceeding the amplitude of 90 degrees is at least required by the time the rotation reaches the synchronized constant speed, which results in drawbacks in self-starting properties.
Furthermore, a reverse rotation preventive mechanism must be always attached to the prior art device since the static central position of the rotor 10 is at two places and the self-starting direction is not defined.